1. The Field of the Invention
The present invention relates generally to the field of fiber optic transceivers. More particularly, embodiments of the present invention relate to methods and devices for attenuating optical signals in a receiver optical subassembly.
2. Related Technology
Fiber optic transceivers are used in a variety of fiber optic applications to convert electrical signals to optical signals and optical signals to electrical signals. One component of a fiber optic transceiver, a transmitter optical subassembly (TOSA), receives an electrical signal, converts the electrical signal to an optical signal, and transmits the optical signal. A second component of a fiber optic transceiver, a receiver optical subassembly (ROSA), receives optical signals and then converts the optical signals to electrical signals.
Detector elements located within ROSAs detect the optical signal that has been received into the ROSA. The detector elements located within ROSAs have limits on the maximum optical power the detector elements can usefully employ. One limit is known as the optical overload limit, or level, and refers to the maximum power of an optical signal that can be successfully processed by a ROSA without the occurrence of bit errors or other degradations beyond the specifications of the ROSA. When the optical overload level is exceeded, the ROSA will cease functioning properly. However, the ROSA will once again function properly when the power level of the optical signal falls back below the overload limit. A second limit, known as the optical damage threshold, refers to a limit on the power of an optical signal which a ROSA is able to withstand. If the power of an optical signal received by the ROSA rises above the optical damage threshold, the ROSA will be permanently damaged.
The optical overload limit and the optical damage threshold are parameters of detectors which may be employed in a ROSA. For example, Avalanche Photodiode Detectors (APDs) are used in ROSAs, particularly in systems where optical amplification is present. APDs use internal gain mechanisms to achieve greater sensitivity than the more common PIN detectors, but tend to do so at the expense of having lower optical overload and damage limits. Networks that employ optically amplified signals are particularly susceptible to damage since the output of optical amplifiers can be as much as 10 to 100 times larger than the overload and damage thresholds of the detectors. In normal configurations, these networks are designed never to expose the detectors to these high power levels. However, it is not uncommon for operator error to result in high powered signals being connected directly to receiver optical inputs, which usually results in destruction of the detector and requires replacement of the entire ROSA.
A common damage mechanism for many detector elements is thermal in nature. Thus, in order to protect a detector from destruction due to high temperatures resulting from excess optical power, it is desirable to limit the time a powerful optical signal is striking the detector to a time significantly shorter than the time necessary to raise the temperature of optical elements beyond a critical point. For the levels of power typically involved (6-20 dBm or 4-100 mW), this time scale is in the range of microseconds (10−6-10−3 seconds).
One technique employed for protecting detector elements from the damaging thermal effect of powerful optical signals relates to controlling the photocurrent through the detector element. Resistors or other current limiting circuits are connected to the detector element to limit the current through the detector element. While such circuitry systems may help to limit one form of detector element destruction due to current heating effects, such systems cannot prevent the direct destruction of the detector element from more powerful optical signals.